A new variant of the H5N1 bird flu — the same one that led to the death of a Louisiana man — has for first the first time been found in infected U.S. dairy cattle.

When it comes to the potential of H5N1 avian flu, otherwise known as bird flu, picking up mutations that may lead to human-to-human spread, that "train has already left the station," warns one infectious disease expert.

A version of the avian influenza virus different from the one rampaging through herds since last spring has been detected in dairy cows in Nevada, USDA says.